Conventionally, most electronic devices have to be connected with power sources (for example power sockets) to acquire electric power through power cables in order to be normally operated. With increasing development of science and technology, a variety of electronic devices are developed toward small size and light weightiness in order to comply with the users' requirements. Moreover, for allowing the electronic device to be easily carried, a built-in chargeable battery is usually installed in the electronic device. Consequently, the electronic device can acquire electric power from the chargeable battery without the need of using the power cable.
For example, in case that the electricity quantity of the chargeable battery within the electronic device is insufficient, the chargeable battery of the electronic device may be charged by using a charging device. Generally, the conventional charging device has a connecting wire. After the conventional charging device is connected with a utility power source and the connecting wire of the charging device is plugged into the electronic device, the electric power from the utility power source can be transmitted to the electronic device through the connecting wire so as to charge the chargeable battery.
However, the applications of the charging device are usually restricted by the connecting wire during the charging process. For example, since the length of the connecting wire of the charging device is limited, the electronic device cannot be operated according to the usual practice or the electronic device cannot be arbitrarily moved. On the other hand, if the conventional charging device has been repeatedly used to charge the electronic device for a long term, the connector of the connecting wire of the charging device is readily damaged because the connector of the connecting wire is frequently plugged into and removed from the electronic device. Under this circumstance, the efficiency of transmitting the electric power is deteriorated. If the connector is seriously damaged, the electric energy cannot be transmitted through the connecting wire.
With increasing development of a wireless charging technology, a wireless charging device for wirelessly charging the electronic device has been introduced into a market in order to solve the drawbacks of using the connecting wire. FIG. 1 schematically illustrates the relationship between a conventional wireless charging device and a conventional electronic device. As shown in FIG. 1, the conventional wireless charging device 1 comprises a casing 10, a power cable 11, a first driving circuit 12, and a transmitter coil 13. In addition, the conventional electronic device 2 comprises a casing 20, a receiver coil 21, a chargeable battery 22 and a second driving circuit 23.
In the conventional wireless charging device 1, the power cable 11 is exposed outside the casing 10 in order to be connected with a utility power source (not shown). Both of the first driving circuit 12 and the transmitter coil 13 are disposed within the casing 10. Moreover, the first driving circuit 12 is connected with the power cable 11 and the transmitter coil 13. When the first driving circuit 12 is electrically driven by the electric power from the utility power source, a driving voltage is provided to the first driving circuit 12 and thus an electric current is generated by the first driving circuit 12. When the electric current flows through the transmitter coil 13, an electromagnetic effect is generated. According to the electromagnetic effect, a magnetic flux is generated by the transmitter coil 13.
In the conventional electronic device 2, the receiver coil 21, the chargeable battery 22 and the second driving circuit 23 are all disposed within the casing 20. The second driving circuit 23 is connected with the chargeable battery 22 and the receiver coil 21. The receiver coil 21 may receive a portion of the magnetic flux from the transmitter coil 13. The portion of the magnetic flux which is received by the receiver coil 21 is further converted into a corresponding electric current. The electric current is transmitted to the chargeable battery 22 in order to perform the charging task.
FIG. 2 schematically illustrates the relationships between the electricity quantity of the chargeable battery, the required load of the electronic device, the magnitude of the current flowing through the transmitter coil and the charging efficiency during the charging process of the chargeable battery of the electronic device as shown in FIG. 1. While the electricity quantity of the chargeable battery 22 is gradually changed from zero to a saturation state, the changes of the required load of the electronic device 2, the magnitude of the current flowing through the transmitter coil 13 and the charging efficiency of the wireless charging device 1 are correspondingly shown in FIG. 2. The required load of the electronic device 2 indicates the magnitude of the required current of the chargeable battery 22.
When the charging process of the chargeable battery 22 starts, the electricity quantity of the chargeable battery 22 is relatively lower. Consequently, the required load of the electronic device 2 is relatively higher. Under this circumstance, the driving power of the first driving circuit 12 is at the high power level to quickly increase the magnitude of the current flowing through the transmitter coil 13. Meanwhile, the transmitter coil 13 generates a larger magnetic flux to the receiver coil 21 of the electronic device 2. Due to the larger magnetic flux, the second driving circuit 23 of the electronic device 2 converts more current to charge the chargeable battery 22. As the electricity quantity of the chargeable battery 22 is gradually increased, the magnitude of the current flowing through the transmitter coil 13 is controlled to decrease. This procedure corresponds to the time interval T11.
When the electricity quantity of the chargeable battery 22 reaches a certain electricity quantity level, the magnitude of the current flowing through the transmitter coil 13 is maintained constant for a certain time interval T12. When the electricity quantity of the chargeable battery 22 approaches saturation, the magnitude of the current flowing through the transmitter coil 13 is controlled to decrease. This procedure corresponds to the time interval T13.
However, during the whole charging cycle of the chargeable battery 22, the charging efficiency of the wireless charging device 1 is not fixed. That is, the ratio of the energy received by the chargeable battery 22 of the electronic device 2 to the energy provided by the wireless charging device 1 is not kept unchanged. As for the current wireless charging technology, the charging efficiency only can be reached to 70%. For example, if the magnitude of the current flowing through the transmitter coil 13 of the wireless charging device 1 is 0.71 A, the most magnitude of the current received by the chargeable battery 22 is 0.5 A. As shown in FIG. 2, the charging efficiency is about 70% during the time interval T12. This is the reason why the magnitude of the current flowing through the transmitter coil 13 is controlled to be 0.71 A for a certain time period after the electricity quantity of the chargeable battery 22 reaches the certain electricity quantity level.
In particular, during the charging process of the chargeable battery 22, the charging efficiency corresponding to the time interval except the time interval T12 is almost lower than 70%. Since the lost electric energy cannot be fully utilized, the waste of the lost electric energy does not comply with the power-saving requirement. In other words, the conventional wireless charging device 1 needs to be further improved.